1. Field of the Invention
The present invention relates to a resin dispersion for cationic electrodeposition and a cationic electrodeposition coating composition including the same, and more particularly to a lead-free cationic electrodeposition coating composition in which the electrodeposited coating film thereof has an epoxy-acrylic double-layered structure to give a high functionality, and in which an organic solvent content is minimized.
2. Description of the Related Arts
Since electrodeposition coating has a good efficiency in application and a high corrosion-resistance and can minimize an environmental contamination because of employing water as a solvent, it is widely used for primer coating of cars, household electric appliances, and industrial machines when comparing with a brush coating or a spray coating. Recently and widely used epoxy-based cationic electrodeposition coating composition includes cationic epoxy resin onto which amine is added and blocked polyisocyanate. Researches at home and abroad on cationic electrodeposition paint are concentrated on paints related to energy conservation, safety, and environmental considerations. Also, researches on electrodeposition coating composition are concentrated on developing highly functional and highly efficient electrodeposition paint having various purposes and characteristics.
Generally, a cationic electrodeposition coating composition includes a tin compound such as dibutyl tin oxide in order to improve a cross-linking reaction between a compound containing hydroxyl functional group with a compound containing isocyanate functional group, along with a lead compound as an anticorrosive pigment in order to improve a corrosion-resistance. However, the lead compound becomes a target of restriction on usage in all over the world as a material inducing an environmental contamination. In July 1996, in Japan, an automobile industrial institute established an independent restriction law named as SHREDDER DUST and suggested each automobile manufacturing company to lessen the utilizing amount of lead in the automobiles. In Europe (EU), a law to restrict on crushing, reclamation, incineration of an automobile containing harmful materials such as lead, cadmium, mercury, etc. after completing the operation of the automobile, will be positively established since 2003. Therefore, with the movements on restricting the environmental pollutants, researches on reducing or removing the harmful materials such as lead from a coated and dried film of the electrodeposition coating composition are actively progressed.
U.S. Pat. No. 5,908,912 (issued to Kollah et al.) discloses a formation of a lead-free dried film by applying a salt of bismuth compound and a carboxylic acid including amine into a pigment paste as a reaction catalyst for preparing a cationic electrodeposition coating composition. In addition, disclosed are the problems of coagulation of the bismuth salt during storage and a lowering of the pH of the electrodeposition coating composition because of the addition of a large amount of an acid in order to applying the bismuth compound into the cationic electrodeposition coating composition.
Among the researches, studies on reducing an organic solvent emission during coating operations are being developed. The motive for these studies is the regulation of emission of organic solvents for the protection of the environment. As examples of the regulations related to the regulation of emission of organic solvents, the regulation for volatile organic compounds (VOC) of the Environment Office in U.S. and the atmospheric purification law (TA-Luft) in Germany can be noted. The former establishes a guideline for the emission amount of the organic solvents from a coating composition, while the latter restricts the total amount of the organic solvents discharged from the paint consumed for coating one car. Since about 20% of the total amount of carbon dioxide discharged into the world is due to the coating industry (for example, organic solvent and combustion by means of a drying oven), the coating industry is a major cause of the environmental pollution. Hence, the coating industry has become the subject of supervision.
Presently, as an electrodeposition coating composition of primer coating, an epoxy cationic electrodeposition paint is mainly used. However, the epoxy cationic electrodeposition paint is rich in organic solvents and these solvents are evaporated from the electrodeposition tank, thereby emitting an offensive odor from a work-piece. Also, these solvents are emitted during curing and drying, so environmental pollution problems occur. In U.S. Pat. No. 6,147,144, (issued to the present inventors), a cationic electrodeposition coating composition in which the amount of the organic solvent is minimized, is disclosed.
Further, even though the conventional epoxy cationic electrodeposition paint has a good corrosion-resistance and a good adherence, this is lacking in properties such as a weather-resistance, a yellowing resistance, etc.